The present invention relates to a discharge machining apparatus, and more particularly to a discharge machining apparatus in which a first electrode and a workpiece serving as another electrode are disposed in confronting relation with an insulative machining solution interposed therebetween, and an electric discharge is generated between the confronting electrodes for machining the workpiece.
FIG. 1 of the accompanying drawings schematically illustrates a conventional discharge machining apparatus. As shown in FIG. 1, an electrode 10 is disposed in confronting relation to a workpiece 14 placed in a machining bath 12 and serving as another electrode with an insulative machining solution 16 interposed between the electrode 10 and the workpiece 14. A machining power supply unit 18 is connected between the electrode 10 and the workpiece 14. The machining power supply unit 18 includes a DC power supply 18a, a switching element 18b for switching on and off a machining current, a current-limiting resistor 18c, and an oscillator 18d for controlling the switching operation of the switching element 18b. The machining power supply unit 18 thus serves to supply the intermittent machining current across an interelectrode gap 20 between the electrode 10 and the workpiece 14.
The machining current, designated by I, is expressed by I=(E-V.sub.g)/R where E is the voltage value from the DC power supply 18a, R the resistance value of the current-limiting resistor 18c, and V.sub.g the voltage across the interelectrode gap 20. The interelectrode voltage value V.sub.g ranges from 20 to 30 V during an arc discharge, is 0 V on short-circuiting, is E V during no discharge, and is 0 V while the switching element 18b is turned off.
The interelectrode gap 20 can be controlled by detecting the interelectrode voltage value V.sub.g and averaging the detected voltage value with a smoothing circuit 22. More specifically, when the interelectrode gap is wide, an electric discharge is less liable to take place and the average voltage V.sub.s is high. When the interelectrode gap is narrow, it tends to be short-circuited or can easily be subjected to an electric discharge, and hence the average voltage V.sub.s is low. Accordingly, the electrode 10 can be controlled to keep the interelectrode gap 20 substantially constant with a hydraulic servomechanism composed of a hydraulic pump 28 and a hydraulic cylinder 30 by comparing the average voltage value V.sub.s with a reference voltage value V.sub.r, amplifying the difference between the voltages with an amplifier 24, and applying the amplified signal to a hydraulic servo valve 26.
In determining whether the workpiece is being properly machined or not in the prior discharge machining apparatus, it has been customary to monitor the average value V.sub.s of the interelectrode voltage V.sub.g. When the average voltage V.sub.s is low, the interelectrode impedance is low, resulting in a short circuit or a continuous arc discharge. When this happens, machined chips or sludge accumulate between the electrodes. Also, an abnormal arc discharge can take place between the workpiece and a body of carbon produced due to thermal decomposition of the machining solution, with the result that the interelectrode impedance is increased. Therefore, it has been impossible to detect a poor condition of the interelectrode gap due to an abnormal arc discharge merely by monitoring the average voltage value V.sub.s.